Hi @knepiosko,
IFSPLSQLAP password is not maintained in application server side in IFSAPP10. Therefore, you won’t see an option to change it either in IFS Installer or IFS MWS Admin Console.
Just make sure that the password for IFSPLSQLAP under plsqlap_environment_tab is the same as the Oracle user password of IFSPLQSLAP. If the password is changed from the "Oracle Users" window in IEE, it will be updated in the plsqlap_environment_tab as well.
I am not sure about your second question.
Hope this helps!
Answer for second question is here:
https://docs.ifs.com/techdocs/Foundation1/010_overview/210_security/010_users/default.htm#Pre-defined_Foundation1_users_
These users must not be mapped to external user registries even if an external user registry (for example ADFS, Azure AD) is used for interactive user authentication